The present invention relates to a glass ceramic article which is decorated with ceramic paints in a screen printing process, as well as to a method of producing the same.
Glass ceramic articles have many applications, for example for temperature-fluctuation resistant (fireproof) kitchen utensils, and in particular as cooking surfaces for kitchen ranges. These articles are generally provided with decorative designs, either for purely aesthetic purpose or for example for marking cooking surfaces at the cooking locations. Ceramic paints based on enamels are used as paints for these decorative designs. They are applied by conventional techniques, for example by screen printing, and subsequently baked. The steps and glass compositions for decorating, dyeing and glazing of glass ceramics are known for example from the German patent documents DE 42 01 286 A1, DE 36 00 109 C2, DE 35 05 922 C1 and DE 34 33 880 C2.
It is known to produce glass ceramic from a nonceramic glass of a composition such as for example disclosed in the patent document EP 0 220 333 B1 by a thermal treatment in accordance with the predetermined temperature-time program (ceramization). In this process the temperatures can reach up to 1000.degree. C.
Also, cooking surfaces for cooking troughs, for example of glass ceramic or special glass, are provided on their upper side with a decorative design of ceramic paints. On the other hand, this decorative design serves for the cooking zone marking and thereby distinguishing between cold and hot regions on the cooking surface. On the other hand, the decoration of the cooking surface performs also an aesthetic function and enables the device manufacturers, by applying of individual decorative features, to produce independent, different product lines.
Until recently, three variants of the decoration of glass ceramic cooking surfaces with screen printing were possible and are on the market:
1. Single paint monochromatic decoration with suitable ceramic paints in hot and cold region with the use of decorative structures of different surface and coating degrees. PA1 2. Single paint-multi paint decoration with suitable ceramic paints only for functional marking of the cooking surfaces, in particular of the cooking zone limiting, but for example also for the residual heat indication or company logo, wherein the glass ceramic surface remains undecorated both in the heating region and in the cold region in a major part. PA1 3. Multi-paint decorations with suitable ceramic decorative paints, wherein during printing a distance must be maintained between different paints to prevent the overlapping and superimposing of the decorative designs which can lead in practical application to peeling of the decorative layer. PA1 4. The German document DE 44 26 234 C1 proposes a solution of this problem by a ceramic article, in particular a glass ceramic cooking surface which is decorated with ceramic paints, wherein on the glass ceramic article there is a first pant layer which covers 1-95% of the glass ceramic surface as a base decorative design, and at least a further paint layer which overlaps the first paint layer at least partially.
With all cooking surfaces having these decorative designs, in the regions with few decorative designs frequently use traces and finger imprints can be found, which are very conspicuous on the continuously smooth, polished surfaces, in particular in exhibition and sales areas.
With glass ceramic cooking surfaces having a scarce decoration, for example only a cooking zone marking, a residual heat indication and a company logo, it is possible that in individual cases, in particular during use of halogen heating bodies and also in the case of strong illumination from above, such as for example in exhibition areas of kitchen studios, it is well known that under certain conditions the "technique below the cooking surface" is visible for the observer or the user, and its aesthetic perception is distorted.
In order to avoid this undesired effect or at least to reduce it, dispersing light coverings are applied. They comprise a dark paint layer, which is applied to the cooking surface lower side so that an observer cannot see through it.
With a decoration in accordance with the DE 44 26 234 C1 a greater part of the outer surface remains uncoated, and during a long utilization of a cooking surface this leads to used traces in form of scratches, to metal wear of the tops and to surface changes of the glass ceramic substrate by unsuitable cleaning means which aggressively and disturbingly act on the cooking surfaces.
The German patent DE 41 29 578C2 discloses a glass disk decorated with a screen printing method, in which a grate-shaped pattern is produced with a combination of different grates, namely a course grate and a fine grate. This is defined in the claims in connection with a portion of the specification starting from column 2, line 45 to column 3, line 30 and illustrated in the drawings.
The German patent DE 40 20 972 C1 describes a screen printing process for glass disks, in which at least two complementary partial surfaces of the decorative design are applied in at least two successive printing steps. The first partial surface is edge-flush along at least one edge with the corresponding first glass disk edge, while the second partial surface is edge-flush along at least one edge with another edge of the glass disk, and mutually complementary partial surfaces of the decorative design overlap at opposite sides at least by a value of the surface tolerances of the glass disk. This is disclosed in the claims in connection with a portion of the specification in column 2, lines 5 to 61, as well as shown in the drawings.
The European patent document EP 0 091 776 A2 discloses a screen printing process for glass disks as described in a portion from page 4, line 8, to page 5, line 25. On page 16, line 13-18 of this patent document there is a hint that also a multi-paint printing is possible.
In the German publication DE 32 24 349 A1 a screen printing oil which is suitable for ceramic paint is disclosed, which is suitable for producing of fine paint grates and relief-shaped printing on ceramic materials, as disclosed in column 3, paragraph 2. This screen printing oil prevents a running of the paint pastes under pressure and provides a contour-sharp printing, so that an additional and overlapping printing of paints with tri-chromatic four-paint printing is possible, as disclosed on page 2, paragraph 2.
The German patent DE 33 20 141 C2 discloses a reproduction method for producing multi-color printing, in which the paint imprint is produced by a plurality of painted flat elements. The image surface is subdivided into identical partial surfaces and each partial surface is subdivided in the adjoining flat elements. This is defined in the claims in connection with a part of the specification from column 3, lines 7 to column 4, line 44. None of the patent documents discloses however a glass ceramic article, in which the mutually corresponding grate-like structural elements are composed on the one hand of structural elements which are positive, and on the other hand of the intermediate spaces between the positive structural elements which are negative.